Emission requirements imposed by environmental legislation on heavy duty diesel engines continue to become higher and higher. Higher peak cylinder pressure is one of the solutions to reduce emissions. To do so, however, stronger material for the cylinder block and the cylinder head is necessary to stand the high pressure of the engine. To use compacted graphite iron could be one of the solutions, however, one must be prepared for higher product cost and lower thermal conductivity, as well as lower damping capacity in the material.
Continued use of grey iron would be positive in many aspects if its strength could be made high enough. The present invention is a contribution toward this target. The effect of nitrogen on the mechanical properties of grey iron has been discussed since 1950's, see for example J. V. Dawson, L. W. L. Smith and B. B. Bach: BCIRA Journal, 1953,4, (12), 540, and/or F. A. Mountford: The influence of nitrogen on the strength, soundness and structure of grey cast iron: The British Foundryman (1966), April, 141-151—all of which are expressly incorporated herein by reference. Increases of nitrogen content on the order of 0.01% or 100 ppm raise the tensile strength by up to 25%. Nitrogen content could be as high as 150 ppm without problems occurring, though the exact nitrogen determination and measurement at that time is discussable.
It has also been showed, for instance in C. Atkin: Nitrogen in iron. Foundry World, Fall, 1 (1979), 43-50 (also expressly incorporated herein by reference), that an increase in nitrogen content from 40 ppm to 80 ppm can increase tensile strength by 10-20% depending on carbon equivalents. Late during this work, it was reported that increases in nitrogen from 40-50 ppm to 140-150 ppm increased tensile strength by 29% without any defect problems, while foundry verification tests were not so successful, P-E. Persson, L-E. Bjorkegren : Gråjärn med forhojda mekaniska egenskaper, Gjuteriforeningen, 20010409 (also expressly incorporated herein by reference). It should be appreciated that all the above data is for separately cast bars.
Although the positive effect was recognized, there is no report of wide application in practical production. Much of the work has been focused on fighting its negative effect, that is, nitrogen in grey iron commercial castings has been considered as a harmful element forming porosity defects in castings, when the nitrogen content is over 90-100 ppm, see J. M. Greenhill and N. M. Reynolds: Nitrogen defects in iron castings. Foundry Trade Journal, 1981, Jul. 16, 111-122, and International committee of foundry technical association: International atlas of casting defects, AFS, 1993 (also expressly incorporated herein by reference). The defect caused by nitrogen is called fissures, blowholes, pinholes or dispersed shrinkage which is seen after machining. The exact allowed levels depend on base chemical composition, other gas contents, casting geometry and solidification rate. Another reason why its positive effect was not widely used could be that the strength requirement on grey iron so far has been easily fulfilled by adjusting carbon equivalent and adding easily controlled alloy elements. However, further increasing the grey iron strength to levels as required in the future using the conventional methods would cause severe castability problems for foundries. A new route is therefore necessary to overcome the castability problem.
Nitrogen content in grey iron melt is usually in the range of 0.004-0.009%, or 40-90 ppm. The exact contents depend on the charge material and the melting process. Melt from cupola with high percentage of steel scrap has higher nitrogen content than melt from electrical furnace and low percentage of steel scrap. Since the content is in such a low level, control of its content is usually ignored in foundry practice, unless some foundries add titanium to the melt to avoid gas porosity in castings.
What is needed, therefore, is a grey cast iron alloy for producing cylinder block and/or cylinder head castings having more strength than present grey cast iron alloys, with good machinability and with a highly controlled level of nitrogen to avoid scrap.